Wide band flexible wave absorber

ABSTRACT

This invention relates to a flexible super-wide band wave absorber composed of a mixture of ferromagnetic and dielectric materials. One form of the wave absorber is composed of a plurality of ferrites having different magnetic resonant frequencies. A further embodiment uses a dielectric material having a high dielectric constant. The wave absorber can be semispherically shaped to form a non-directional wave absorber.

llnited States Patent [191 Suetalte et a1.

[ Aug. 21, 1973 WIDE BAND FLEXIBLE WAVE ABSORBER [76] Inventors:Kunihiro Suetake, No. 11, IO-ban,

Minami 3-chomc, Meguro-ku, Tokyo; Toshimitsu Musha, c/o Tokyo Instituteof Technology, No. 12-1, 2-chome, Ohokayama, Meguro-ku, Tokyo; YoshiyukiNaito, No. 261-44, Suenaga, Kawasaki-shi, Kanagawa-ken, all of Japan 22Filed: Apr.5, 1971 211 Appl. No.: 130,937

[52] US. Cl 343/18 A [51] Int. Cl. H0111 17/00 [58] Field of Search343/18 A [56] References Cited UNITED STATES PATENTS 3,526,896 9/1970Wesch 343/18 A 3,038,551 6/1962 McCoy et a1. 343/18 A 3,315,260 4/1967Wesch 343/18 A 3,568,196 3/1971 Bayrd et a1. 343/18 A 2,292,444 8/1942Haydon et a1. 343/18 A 3,307,186 2/1967 Straub 343/18 A PrimaryExaminerBenjamin A. Borchelt Assistant Examiner-G. E. MontoneAttorney-Leonard l-l. King 5 7] ABSTRACT This invention relates to aflexible super-wide band wave absorber composed of a mixture offerromagnetic and dielectric materials. One form of the wave absorber iscomposed of a plurality of ferrites having different magnetic resonantfrequencies. A further embodiment uses a dielectric material having ahigh dielectric constant. The wave absorber can be semisphericallyshaped to form a non-directional wave absorber.

13 Claims, 18 Drawing Figures I NVENTORS Pmmmwm ma 3.754.255

SHEET 1 BF 7 FIG. 1

2 c m H m l Y b 2 2 W W 7///% 1 FIG. 2a

KUNIHIRO SUETAKE BY TOSHIMITSU MUSHA YOSIIIYUKI NAITO minimum ma3.754.255

SHEET 3 0F 7 FIG. 5C1

PATENTEUAUGZI ms SHEU 5 0f 7 fre qd PMENTEDAIIBZI I915 3,754,255 SHEET 7OF 7 IG.1OQ

FIG. 10b

BACKGROUND OF THE INVENTION Wave absorbers are an important tool forresearch involving radio waves. They are widely used in many kinds ofapplications of radio engineering. One of the most important needs ofusers of wave absorbers is a thin wave absorber. This problem isuniversal to all kinds of waves from short to long wave lengths. It isgenerally known that the thickness of the absorbing wall has to be atleast a half of the length of the wave to be absorbed. Accordingly, inorder to absorb waves of lower frequency, which have longer wavelengths, conventional wave absorbers made of only dielectric materialmust be rather thick. For instance, a thickness of 150 cm. is necessaryfor absorbing a 100 MHz wave. It has been found that by usingferromagnetic ferrites, the thickness of the absorbing wall can be muchreduced. Using ferrites, the thickness of the absorbing wall can beabout 8 mm for almost all kinds of wave lengths, which represents agreat reduction compared with the dielectric wall of the prior art.However, for some applications even this is too thick. Distortedtelevision images resulting from the interference of reflections fromwater tanks, towers and high buildings, is a current problem which canbe solved by coating the outer surface of the buildings with a waveabsorber. However, the above mentioned 8 mm absorber is too thick forthis purpose. Accordingly, thinner absorbers are demanded.

An object of this invention is to provide a thin wave absorber for thewide band of electromagnetic waves.

Another object of this invention is to provide a wideband wave absorbercomposed of a mixture of one or more kinds of ferrite powders'which havedifferent magnetic resonant frequencies and a dielectric binder such asrubber or resin.

A further object of this invention is to provide a wave absorbantcoating for a structure.

These and other objects, features and advantages of the invention will,in part, be pointed out with particularity and will, in part, becomeobvious from the following more detailed description of the invention,taken in conjunction with the accompanying drawings, which form anintegral part thereof.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a perspective view of a waveabsorber;

FIGS. 2 a, b and c are cross-sectional views of various wave absorbers;

FIG. 3 is a Smith chart showing the characteristics of the waveabsorber;

FIGS. 4 a, b, c and d are plane views of wave absorbers made to fitparticular shapes;

FIGS. 5 a and b show the relation between the imaginary part of thecomplex specific magnetic permeability of absorbers having differenttypes of ferrite compositions;

FIG. 6 shows the relation between the thickness of the absorber and thefrequency of the wave to be absorbed;

FIG. 7 shows the resultant characteristics of a wave absorber composedof a mixture of various ferrites;

FIG. 8 is a plan view of an absorber composed of various kinds offerrites;

FIG. 9 a shows the relation between the incident angle and the absorber,and FIG. 9 b shows the reflection ratio for the absorber of FIG. 9 a;and

FIGS. 10 a and b are plan and sectional views, respectively, ofnon-directional absorbers.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS The principles on which thisinvention is based relate to the use of the electromagnetic waveabsorbing member described in U. S. Pat. No. 3,460,142.

This most important feature of this invention is the utilization of theproperties of ferrites wherein specific magnetic permeability isrepresented by it, I- r ii/ 'r The effective part, ,u',-, is greatlydecreased and the loss part, 11",, is greatly increased in theneighborhood of the resonant frequency, and u, then decreases as thefrequency increases above the resonant one.

As shown in FIG. I, a ferrite plate ll, whose thickness is 11.1 2 1m",-where A is the wave length, is prepared and a metal plate 2 made fromconductive material is fixed to one surface of the plate I by means ofan adhesive of synthetic resin or the like. The electromagnetic wave 3coming from the other surface is absorbed throughout a wide band offrequency. For example, if u", is 20 and the ratio of l is 1/120, then Iis less than 2.5 cm for a wave length of 3 m which corresponds to afrequency of MHz. This represents a significant improvement over theconventional dielectric wall which would need a thickness of 1.5 to 2 mto absorb the same wave.

As shown in FIG. 2, two or more plates 1a, 1b made of various ferritescan be placed in contact with each other or spaced by gap 4. Thethickness of the ferrite plates is chosen as described hereinafter. Inthis case, even if the individual characteristics of each ferrite platedoes not satisfy the required conditions, a wide band wave can still beabsorbed by a combination of the various ferrite plates.

There are, however, some problems in using these ferrite absorbers,namely: I

1. A large plate cannot be easily produced because a thin ferrite platemade by a sintering process is often curved.

2. Ferrite plates cannot be placed directly on a curved surface withoutgaps resulting because the plates are like rigid tiles and cannot beshaped to conform to the surface. The gaps between the plates interferewith the absorption properties of the absorbing wall.

Thus, it would be desired to have a ferrite absorber capable ofabsorbing a wide band of electromagnetic waves whose thickness is lessthan 8 mm and being flexible. According to this invention, it ispossible to achieve such a ferrite absorber.

In accordance with this invention, ferrite powder is mixed withdielectric material such as rubber and flexible plates are made of thismixture. If the mix ratio is properly chosen, flexible plates having thesame magnetic properties as rigid plates can be obtained.

When the powder of Mg-Cu-Zn ferrite is mixed with rubber in the ratio ofw 1 (w parts of the ferrite to one part of rubber), the frequency of thewave to be absorbed is determined by w. The magnetic permeability ofMg-Cu-Zn ferrite is about 200 and it absorbs a wave of 500 MHZ. But ifrubber is added to this ferrite in a ratio wherein w is 3, 4.5 or 10,the frequency of the wave absorbed is 4,000, 2,200 or 1,500 Ml-Iz,respectively. When Mn-Zn ferrite whose magnetic permeability is about5,000 is added to rubber wherein the ratio of w to rubber is :1,thefrequency is 500 MHz. As is clear from the above example, if a ferriteof higher permeability is mixed with rubber, the same property as pureferrite of lower permeability can be obtained.

Curves a and b of FIG. 3, respectively, show the properties of aflexible wave absorbing wall of this invention and those of a pureferrite plate of the prior art. From the standard Smith chart of FIG. 3the relation between the input impedance and the thickness of the samplecan be seen.

In the above examples, thermoplastic resins such as butyl-rubber,neoprene or pyparone can also be used as the dielectric material. Theproduct is flexible, its thickness can be easily changed as the mixpasses through a calender and a continuous long plate can be produced.The plate can be cut in any desired shape so that the wave absorbingwall can be fixed onto the surface of any configuration without any gaptherebetween.

FIG. 4 shows the cross-sectional view of examples of the many shapes ofwave absorbers according to this invention. FIGS. 4 a, b, c and d showrespectively a stepped connection, a tapered connection, a waveformconnection and an insert connection. It is understood that in adaptingthe absorber of this invention to any surface without a gap, if theabsorber is to be used as an absorbing wall, a metal plate is fixed onthe back surface of the absorber.

In addition to the single ferrite mixture hereinbefore described, two ormore kinds of ferrites whose magnetic resonant frequency are differentfrom each other can be mixed so that any desired wide bandcharacteristics and desired central working frequency can be obtained.The ferrites could be mixed with the dielectric material.

FIGS. 5a and b show the characteristic curves of the imaginary part ofthe complex specific permeability as a function of the frequency f. FIG.5 a shows these characteristics for absorbing plates made of the mixtureof rubber and an individual one of the ferrites herein referred to as A,B and C. FIG. 5 b shows the characteristic of a single absorbing platemade of the mixture of rubber and a combination of the ferrites A, B andC. It can be seen that the imaginary part 11.", of the permeabilityvaries inversely to the frequency in a wide band.

For example, H ferrite composed of 26.01 percent MnC0 2.06 percent CuO,17.39 percent ZnO and 54.53 percent Feg03; A ferrite composed of 16percent MO, 34 percent ZnO and 50 percent F6903; and M ferrite composedof 25.2 percent MgO, 8.1 percent CuO 18.6 percent ZnO and 47.3 percentFe tl were prepared. H A and M are trade marks of the TDK ElectronicsCompany LTD. The specific band is defined by the ratio B/f where B isthe band width and f is the central frequency. When the value of thespecific band B/f is large, the characteristic of the ferrite is good.

The specific bands of ferrites H A and M are 14.3 percent, 22.6 percentand 22.4 percent, respectively. But the mixtures of A and M in the ratioof 1 1; M and H in the ratio of 1 1; A and M in the ratio of 2 l; and Aand M in the ratio of 1.5 1 will have specific bands of 62 percent, 68.1percent, 31 percent and 34.6 percent, respectively.

Thus, a flexible wave absorber can be made composed of the mixture of aplurality of ferrites whose magnetic resonant frequencies are differentfrom each other and which exhibits wide band characteristics.

A further variation of this invention is to make a wave absorber bysintering the mixture of a dielectric powder having a high dielectricconstant with the ferrite powder. An example of such material consistsof Ba Ti 0 74 mol Ba Zr O 14 mol Ca Ti 0;, 9 mol Mg Ti 0, 3 mol to whichis added 0.2 percent by weight of Mn C0 The material is mixed andshaped. The product obtained from this process is presintered at 1,1 Cfor 2 hours, crushed and again shaped and sintered at l,350 C for 2hours. The resultant sintered material is again crushed and the powdereddielectric material thus obtained is mixed with ferrite. This process ismost preferable. However, presintered dielectric material may be usedand mixed with the ferrite without the additional sintered process.

FIG. 6 shows the characteristics of this type of absorber wherein thethickness is plotted as a function of frequency for various dielectricconstants. It can be seen that the thickness necessary for absorbing awave at a particular frequency is dependent upon the specific dielectricconstant er. As is clear from FIG. 6, by using dielectric material whosedielectric constant is more than 5, the thickness can be remarkablydecreased. By using a dielectric substance having a high dielectricconstant an absorbing wall can be had with very small thickness to theextent that it can be made into a paint. Ghost images can then beavoided by painting the outer surfaces of buildings with the wallabsorber and thereby eliminate interference.

In addition to combining various ferrites into a mixture, the ferritescan be combined in other ways. For example, three kinds of ferrites,herein referred to as A, B and C, were prepared. The characteristic ofthe imaginary part of the complex specific permeability 1.1.", as afunction of the frequency for each of the ferrites A, B and C are shownrespectively in FIG. 7. These ferrites were disposed in a randomarrangement as shown in FIG. 8. In FIG. 8 the areas S,,, S and Soccupied by the materials A, B and C can be determined by the followingequations:

WhereA A and A are the matching frequencies of ferrites A, B and C,respectively.

The resultant characteristic of the absorber comprising ferrites A, Band C disposed as shown in FIG. 8 is shown by curve D in FIG. '7. As isclear from curve D,

the resultant absorber is effective over a wide range of frequencies.

In this example, the thickness of the various ferrites was uniform andthe surface of the absorber of FIG. 8 was flat.

An application of the flexible wave absorber of this invention will nowbe described.

In general, a wave absorber is made of a ferrite plate 1 and a metalplate 2 as shown in FIG. 9a, and a good absorbing property can beobtained for waves arriving perpendicularly to the face of the absorber,but not for waves arriving obliquely to it.

As shown in FIG. 90 wherein the angle 0 represents the incident angle ofthe electromagnetic waves onto the absorber, FIG. 9b shows the relationbetween the absolute value of the reflection ratio 8 and the incidentangle 0. As can be seen the reflection ratio 8 is 10 percent when theincident angle is 33" though it is zero when the wave comesperpendicularly to the face of the absorber and 0 is zero.

The wave absorber shown in FIG. 10 has a semispherical surface, andtherefore, whatever direction the wave comes from, a part of theabsorber is perpendicular to the incident wave. The absorber of FIG. 10is composed of a metal plate 11, a semispherical, flexible absorbingferrite 13, a semispherical -metal plate adapted to the back surface ofthe ferrite 14.

The absorber shown in FIGS. 10 a and 10b is practically produced byaffixing the ferrite to the semispherical metal plate 14, and goodabsorbing property is obtained for the incident wave in all directionsso that'a non-directional absorber is provided.

Ferrites, the mixture of ferrite and rubber, and the mixture of ferriteand dielectrics can be used as the absorbing material of the absorber ofFIGS. 10a and 10b.

There has been disclosed heretofore the best embodiment of the inventionpresently contemplated. However, it is to be understood that variouschanges and modifications may be made by those skilled in the artwithout departing from the spirit of the invention.

We claim:

1. An electromagnetic wave absorber comprising, a contiguous thinferrite plate composed of a plurality of ferrite materials whosespecific magnetic permeability is represented by the formula ZF-p',ju",. where p,

where S, is the surface area of one of the sections A, A is the matchingfrequency of the ferrite material of that section and n is the totalnumber of ferrite materials comprising the ferrite plate.

4. A wave absorber as in claim 2 wherein said dielectric material has ahigh specific dielectric constant.

5. A wave absorber as in claim 1 wherein each of said ferrite materialsis further comprised of a mixture of at least two ferrites havingdifferent magnetic resonant frequencies.

6. A wave absorber as in claim 4 wherein said specific dielectricconstant is more than five.

7. A wave absorber as in claim 2 wherein said dielectric material isfurther comprised of a ferroelectric material and a dielectric binder.

8. A wave absorber as in claim 1 wherein each of said ferrite materialsis of uniform thickness and said ferrite plate isflat.

9. A wave absorber as in claim 2 wherein said dielectric material isrubber.

10. A wave absorber as in claim 2 wherein said dielectric material is asynthetic resin.

11. A wave absorber as in claim 2 wherein said mixture is in apredetermined ratio said ratio dependent upon the frequency of the waveto be absorbed.

12. A wave absorber as in claim 1 wherein said absorber has asemispherical shape.

13. The wave absorber as in claim 1 wherein said absorber is in the formof a plurality of semispheres.

i i i

2. A wave absorber as in claim 1 wherein each of said ferrite materialsis composed of a mixture of ferrite and dielectric material.
 3. A waveabsorber as in claim 1 wherein the sections have a surface areadetermined by the formula
 4. A wave absorber as in claim 2 wherein saiddielectric material has a high specific dielectric constant.
 5. A waveabsorber as in claim 1 wherein each of said ferrite materials is furthercomprised of a mixture of at least two ferrites having differentmagnetic resonant frequencies.
 6. A wave absorber as in claim 4 whereinsaid specific dielectric constant is more than five.
 7. A wave absorberas in claim 2 wherein said dielectric material is further comprised of aferroelectric material and a dielectric binder.
 8. A wave absorber as inclaim 1 wherein each of said ferrite materials is of uniform thicknessand said ferrite plate is flat.
 9. A wave absorber as in claim 2 whereinsaid dielectric material is rubber.
 10. A wave absorber as in claim 2wherein said dielectric material is a synthetic resin.
 11. A waveabsorber as in claim 2 wherein said mixture is in a predetermined ratiosaid ratio dependent upon the frequency of the wave to be absorbed. 12.A wave absorber as in claim 1 wherein said absorber has a semisphericalshape.
 13. The wave absorber as in claim 1 wherein said absorber is inthe form of a plurality of semispheres.